This invention relates to accelerometers, and is more particularly, but not exclusively, concerned with accelerometers for use in down-hole instrumentation for surveying a borehole.
U.K. Patent Specification No. 1,492,142 discloses an accelerometer comprising a housing defining a chamber, a magnetizable fluid within the chamber, a permanent magnet magnetically suspended within the chamber by the magnetizable fluid with its poles oriented along a displacement axis and displaceable from a null position along the displacement axis by an applied force, and sensing means for detecting displacement of the permanent magnet along the displacement axis and for supplying an electrical signal indicative of the applied force.
Such an accelerometer requires to be calibrated prior to use, and operates satisfactorily under normal conditions. However, it is found that the required calibration of the accelerometer can tend to drift under the hostile conditions of high temperature and vibration encountered down-hole, and this can lead to inaccuracy in measurement. Such drift can occur even where the accelerometer has been subjected to heat stressing prior to use.
The applicants have examined the possible causes of such drift, and have formulated what they believe to be a reasonable explanation for it, as follows. The magetizable fluid comprises a plurality of magnetizable particles in colloidal suspension within a suspending medium, with the result that the fluid as a whole appears to be magnetizable. Prior to use of the accelerometer down-hole, certain of the magnetizable particles tend to migrate under the effect of magnetic attraction towards the outer surface of the permanent magnet so as to form an increased concentration of particles in the vicinity of the poles of the magnet. The accelerometer is calibrated under these conditions. However, when the accelerometer is used down-hole, the effect of the high temperatures encountered will be to reduce the viscosity of the suspending medium of the magnetizable fluid, and the effect of the high vibration and temperature encountered, combined with the reduced viscosity, will cause a change in the concentration of the magnetizable particles in the vicinity of the poles of the magnet. As a result the conditions are changed as compared with those under which the accelerometer was calibrated, and an error is introduced into the measurements. However it is to be appreciated that the above is only put forward as a tentative explanation of such errors, as it is not possible to predict with any certainty at this stage precisely what mechanisms are responsible for the calibration drift, or indeed whether different mechanisms might not be responsible under different conditions.
It is an object of the invention to provide an accelerometer which is capable of improved performance under hostile conditions.